Re: [Qemu-devel] [PATCH 02/10] qemu-thread: introduce QemuLockCnt

[Paolo Bonzini]

On 11/01/2017 16:48, Fam Zheng wrote:
> On Wed, 01/04 14:26, Paolo Bonzini wrote:
>> +For example, QEMU uses QemuLockCnt to manage an AioContext's list of
>> +bottom halves and file descriptor handlers.  Modifications to the list
>> +of file descriptor handlers are rare.  Creation of a new bottom half is
>> +frequent and can happen on a fast path; however: 1) it is almost never
>> +concurrent with a visit to the list of bottom halves; 
> 
> Isn't it common that thread A is busy notifying thread B with BH, and thread B
> is busy processing the notification? In that case creation and visiting the BH
> list are concurrent.

For multi-threaded cases you may can create the BH just once
(thread-pool does it, and aio_co_schedule will be there in the next part
of the work to simplify "remote" qemu_coroutine_enter).

The case where thread A creates the bottom half does occur with rbd.c,
but then, the next point applies:

>> 2) it only has
>> +three instructions in the critical path, two assignments and a smp_wmb().
>> +
>> +/**
>> + * qemu_lockcnt_dec: possibly decrement a QemuLockCnt's counter and lock it.
> 
> s/qemu_lockcnt_dec/qemu_lockcnt_dec_if_lock/

Oops, I'll wait for full review and send v4.

>> + * @lockcnt: the lockcnt to operate on
>> + *
>> + * If the count is 1, decrement the count to zero, lock
>> + * the mutex and return true.  Otherwise, return false.
>> + */
>> +bool qemu_lockcnt_dec_if_lock(QemuLockCnt *lockcnt);
>> +
>> +/**
>> + * qemu_lockcnt_lock: lock a QemuLockCnt's mutex.
>> + * @lockcnt: the lockcnt to operate on
>> + *
>> + * Remember that concurrent visits are not blocked unless the count is
>> + * also zero.  You can use qemu_lockcnt_count to check for this inside a
>> + * critical section.
>> + */
>> +void qemu_lockcnt_lock(QemuLockCnt *lockcnt);
>> diff --git a/util/lockcnt.c b/util/lockcnt.c
>> new file mode 100644
>> index 0000000..78ed1e4
>> --- /dev/null
>> +++ b/util/lockcnt.c
>> @@ -0,0 +1,113 @@
>> +/*
>> + * QemuLockCnt implementation
>> + *
>> + * Copyright Red Hat, Inc. 2015
> 
> 2015, 2016?

Or 2017 even. :)

Paolo

>> + *
>> + * Author:
>> + *   Paolo Bonzini <address@hidden>
>> + */
>> +#include "qemu/osdep.h"
>> +#include "qemu/thread.h"
>> +#include "qemu/atomic.h"